Absorbent and superabsorbent hydrogels are typically obtained from vegetable or animal proteins, mixtures of vegetable and animal proteins, cellulose, hemicellulose, saccharides (typically C5 and C6 sugars present in plant derived proteins), and polysaccharides. In general, hydrogels are utilized in industrial dewatering applications; maintaining moisture retention in soils, especially in regions experiencing low rainfall; remediation of heavy metal contaminated soil, based on complexation of heavy metal cations with polycarboxylic acids; in control of wildfires, based on application of water-saturated hydrogels; and in diaper and other water/urine absorbing applications.
The combined market for hydrogels is over one billion pounds per year in the U.S. and about 2.5 times that globally, with a growth rate in both markets of approximately 3% per year. Most water-soluble polymers and hydrogels are currently prepared from petroleum-based monomers. Petroleum-based feedstocks for hydrogels include polyvinyl alcohol, polyacrylic acid, polyacrylamide and maleic anhydride/butylene copolymers.
Existing hydrogels are characterized by incorporation of a relatively high percentage of water solubilizing groups such as carboxylic acid, amide and alcohol groups. High performance hydrogels are further characterized by appropriate levels of cross-linking as illustrated in polysuccinimide cross-linked with polyaspartic acid and cross-linked copolymers of maleic anhydride and maleimide.
In prior USDA-funded work, soluble soy protein from soy protein isolate has been converted to high performance hydrogels by using ethylenediaminetetraacetic dianhydride (EDTAD) as the key reagent to provide protein cross-linking and introduce pendant carboxylic acid groups by reaction with lysine amine groups. See D. C. Hwang and S. Damodaran, J. Appl. Polymer Sci. 1996, 62, 1285. See, also U.S. Pat. No. 5,847,089 to Damodaran & Hwang and U.S. Pat. No. 6,310,105 to Damodaran. However, this work has economic disadvantages in that the price of soy protein isolate is fairly expensive and EDTAD is priced at $139/50 g (equivalent to $1262/pound) in the 2007-2008 Aldrich catalog. Currently, EDTAD is not available at bulk scale.
Work by Yang, et al. describes methods for grafting methacrylic acid to the protein in soy protein isolate when using persulfate radical initiators. See C. Yang, et al., Journal of Applied Polymer Science 2006, 102, 4023-4029. However, Yang et al., does not employ acrylate based cross-linkers that are necessary to convert soy protein to a hydrogel. Methods for grafting acrylic monomers to cellulose using ceric ammonium nitrate (Ce(IV)) initiated radical grafting have been described. Although the grafted cellulose products did have somewhat enhanced water absorption compared to non-modified cellulose, none of these products were indicated as having hydrogel properties. See E. Rezai and R. R. Warner, Journal of Applied Polymer Science 1997, 65, 1463-1469; and V. Jain, H. Xiao, and Y. Ni, Journal of Applied Polymer Science 2007, 105, 3195-3203. Hydrogel production using methods for grafting acrylic acid to artemesia seed gum (a natural high molecular weight polysaccharide) using a microwave oven have also been described. See J. Zhang, et al., Journal of Macromolecular Science 2007, 44, 881-885.